The proposed epidemiological and experimental studies of breast cancer will investigate host, agent and environmental factors in order to trace the etiology and pathogenesis of this disease. We will investigate host factors from the perspectives of genetics, cytopathology, physiology and biochemistry. We will analyze carcinogenic agents which may be secreted into the breast fluids of women and the role of these agents in the initiation and promotion of breast cancer. We will explore certain nutritional components of the environment and their possible role in breast cancer etiology. We will also investigate health practices of women with regard to early breast cancer detection. These studies will hopefully allow us to suggest new hypotheses for the etiology of breast cancer and to predict more accurately the risk of breast cancer for asymptomatic women. Elucidation of processes of initiation and pathogenesis of breast cancer could lead to the development of preventive programs against this disease.